In forming a network using the optical signal modulated by the data signal, it is necessary to monitor the quality of the optical signal propagating through the network with the optical signal monitoring apparatus.
Generally, the optical signal monitoring apparatus obtains information on a waveform of a signal on a transmission path, and computes a value indicating the signal quality from the waveform information and supplies the computation result. Therefore, in the optical signal monitoring apparatus, it is extremely difficult to obtain the waveform information by a photodetector directly receiving the optical signal which is modulated by a data signal having a bit rate very high as tens of Gb/s.
In a conventional optical signal quality monitoring apparatus, an equivalent time sampling method is adopted to obtain the information on the waveform of the high-speed optical signal.
In the equivalent time sampling method, sampling is performed to an optical signal P in which the same waveform is repeated at a period Ta as shown in (a) of FIG. 11 using a sampling pulse S having a period of Ts=N·Ta+ΔT which is slightly longer than N multiplications (N is an integer) of the period Ta by a time ΔT as shown in (b) of FIG. 11, thereby obtaining instantaneous amplitude values (instantaneous intensity) at positions shifted from each other by ΔT in the repetitive waveform of the optical signal P as shown in (c) of FIG. 11.
A waveform P′ drawn by an envelope curve connecting the obtained instantaneous amplitude values is one in which the waveform of the optical signal P is enlarged by a factor of Ts/ΔT on a temporal axis. The waveform P′ retains the waveform characteristics of the original optical signal P.
Accordingly, a probability distribution of an amplitude indicating one of binary levels and an amplitude indicating another of the binary levels is obtained for the waveform information obtained by the equivalent time sampling, and a Q value indicating the signal quality can be obtained by computation of a standard deviation of the probability distribution.
For example, Patent Document 1 discloses a technique of performing the equivalent time sampling to the optical signal to compute the Q value indicating the signal quality from the obtained waveform information.
Patent Document 1: Japanese Patent No. 3796357
However, in the technique disclosed in Patent Document 1, only an intensity-modulated optical signal can be monitored by the data signal, and it is impossible to monitor a modulation state of an optical signal phase-modulated by a Differential Phase-Shift Keying (DPSK) method or a Differential Quadrature Phase-Shift Keying (DQPSK) method whose use on an optical network are being studied.